The present invention relates generally to thermostatic expansion valves for air-conditioning systems, and more particularly to thermostatic expansion valves for vehicle air-conditioning systems.
In a typical vehicle air-conditioning system, refrigerant is compressed by a compressor unit driven by the automobile engine. The compressed refrigerant, at high temperature and pressure, enters a condenser where heat is removed from the compressed refrigerant. The refrigerant then travels through a receiver/dryer to an expansion valve. The expansion valve throttles the refrigerant as it flows through a valve orifice, which causes the refrigerant to change phase from liquid to a saturated liquid/vapor mixture as it enters the evaporator. In the evaporator, heat is drawn from the environment to replace the latent heat of vaporization of the refrigerant, thus cooling the environmental air. The low pressure refrigerant flow from the evaporator returns to the suction side of the compressor to begin the cycle anew.
The high pressure refrigerant flow through the expansion valve must be regulated in response to the degree of superheat of the refrigerant flow between the evaporator and suction side of the compressor to maximize the performance of the air-conditioning system. The superheat is defined as the temperature difference between the actual temperature of the low pressure refrigerant flow and the temperature of evaporation of the flow. One type of device used to remotely sense the degree of superheat of the flow is a feeler bulb. The feeler bulb is positioned in contact with the pipe carrying the low pressure refrigerant. A pressure carrier extends from the feeler bulb to a valve element in the expansion valve to regulate refrigerant flow between the evaporator and the condenser.
Another, more recent device to sense the degree of flow superheat is the block-type (“bulbless”) thermostatic expansion valve. Bulbless thermostatic expansion valves typically include a power element comprising a diaphragm mounted between a domed head and a support cup on the valve body. A “charge” is located within the head chamber defined by the domed head and one (upper) surface of the diaphragm. The support cup and the other (lower) surface of the diaphragm define a diaphragm chamber with the body of the expansion valve. A pressure pad is located against the lower surface of the diaphragm, and extends downwardly through the diaphragm chamber and through an opening in the valve body into the refrigerant flow path from the evaporator outlet. A valve stem is connected to the pressure pad, and extends further downwardly through a bore in the valve body to a valve element modulating a valve orifice between a first port in the valve body (to the condenser) and a second port in the valve body (to the evaporator).
A typical valve element for a thermostatic expansion valve includes a ball located in a ball retainer or ball seat which is biased by a spring against the valve orifice between the condenser port and the evaporator port. It is also known to support the ball directly against the end coil of the spring, and to use a cone-shaped element instead of a ball. In any case, the valve stem engages the ball and urges the ball away from the valve orifice in response to movement of the diaphragm. The spring is held in place by a gland or spring seat, which can be screwed into the passage leading to the outlet in the body. Adjustment of the axial location of the gland (such as by screwing the gland into or out of the valve body) adjusts the spring force on the valve ball, and hence adjusts the flow through the valve. The ball retainer and gland are typically formed from brass, and are otherwise separated from each other by the spring.
To control the refrigerant flow, the diaphragm in the power element senses the refrigerant condition exiting the evaporator and compensates the flow rate to the evaporator by opening or closing the valve orifice. In certain bulbless valves, the pressure pad is thermally conductive, and as the refrigerant from the evaporator outlet passes around the pressure pad, heat energy is transferred by conduction through the pad to the refrigerant charge in the head chamber above the diaphragm valve. A portion of the diaphragm surrounding the pressure pad is typically also exposed to and in direct contact with the refrigerant from the evaporator outlet. Refrigerant pressure from the evaporator outlet against the diaphragm along with the force of an adjustment spring on the valve element tends to close the valve, while pressure from the charge tends to open the valve. The balance of forces across the diaphragm along with the spring constant of the diaphragm determine the deflection of the diaphragm and hence the opening of the expansion valve orifice between the condenser and evaporator. The diaphragm deflects as appropriate to maintain a balance between these forces.
The pressure of the charge in the head chamber above the diaphragm valve is governed by the pressure/temperature relationship of the gas(es) in the charge. The pressure pad ideally becomes the same temperature as the refrigerant flowing through the valve, and along with the refrigerant in direct contact with the diaphragm, the temperature of the charge generally follows the temperature of the refrigerant exiting the evaporator. Fukuda, U.S. Pat. No. 6,223,994; Proctor, U.S. Pat. No. 3,691,783; Treder, U.S. Pat. No. 3,537,645; and Orth, U.S. Pat. No. 3,450,345, show and describe examples of bulbless expansion valves such as described above.
One of the issues that designers of bulbless expansion valves have had to address is the sensitivity of the valves to external conditions. For example, in vehicle engine compartments, the expansion valve can be subject to substantial thermal transients which can detrimentally affect the operating characteristics of the valve and hence impact the performance of the system. This is believed due in part to the heat energy provided by conduction from the ambient surrounding the valve through the power element to the charge in the head chamber. Heat energy from a vehicle engine can increase during engine operation, which can cause the temperature and pressure within the head chamber to increase irrespective of the damping effect of the pressure pad and the direct contact of the refrigerant with the diaphragm. In this situation, the valve can open more than desired, which can allow excess refrigerant to flow through the valve and consequently, liquid refrigerant to flow to the compressor. Liquid refrigerant flow to the compressor can be detrimental for proper compressor function, and affect the performance of the compressor and hence the over-all performance of the air-conditioning system.
Attempts have been made to closely thermally couple the charge to the refrigerant, to in effect, use the refrigerant as a heat sink. One known technique is to use a highly-conductive pressure pad, formed from e.g., aluminum, to cause the valve to be less susceptible to external conditions—and more closely follow the temperature changes in the refrigerant.
However, in so doing, the valve can become overly-sensitive to temperature changes in the refrigerant. An aluminum pressure pad, for example, almost instantaneously transfers heat energy between the refrigerant and the charge. This can also be detrimental, as the valve can then be susceptible to hunting. That is, there a lag time between a superheat transient in the low pressure refrigerant flow, and a compensating regulation in the expansion valve. The valve tends to over-compensate in both directions. As can be appreciated, this can also negatively effect the performance of the air-conditioning system.
It is believed one of the primary reasons for such sensitivity and hunting problems is because conventional wisdom is to form the pressure pad from aluminum. While aluminum is an inexpensive, easily-workable material; it does not have sufficient thermal mass to absorb thermal transients in the refrigerant and prevent (or at least reduce) the hunting of the valve.
Techniques are known which have attempted to address the hunting issue in valves with aluminum pressure pads. One technique is to add an insulator such as a plastic sleeve around the pin of the pressure pad to reduce the rate of heat transfer. This can improve the “hunting” of the valve, but it adds manufacturing complexity.
Thus, applicants believe heretofore there has been a trade-off between achieving appropriate sensitivity to the temperature of the refrigerant vapor in the valve, and smooth operation of the valve, so as to maintain a proper control of the refrigerant flow in a refrigeration system. Applicants therefore believe there is a demand in the industry for a simple thermostatic expansion valve which overcomes the above issues, that is, for a valve which has a reduced sensitivity to external temperature transients, particularly temperature fluctuations caused by engine heat in an engine compartment, and which is more appropriately responsive to temperature changes in the refrigerant from the evaporator.